This invention relates to an array of disk drives accessed by a host computer, more particularly to the channel arrangement of such an array.
Disk drives are widely used for secondary storage of information in computer systems, and many computer systems have an array of disk drives which are connected to a host computer by one or more channels. One advantage of such an array is that it can provide extra reliability by storing data in a redundant form. In particular, it has been shown that a redundant array of inexpensive disk drives (referred to as a RAID) can provide higher reliability at lower cost than a single large, expensive disk drive.
RAID schemes have been classified into five levels: a first level in which the same data are stored on two disks (referred to as mirrored disks); a second level in which data are bit-interleaved across a group of disks, including check disks on which redundant bits are stored using a Hamming code; a third level in which each group has only a single check disk, on which parity bits are stored; a fourth level that uses sector interleaving and a single check disk per group; and a fifth level that uses sector interleaving and distributes the check information evenly over all disks in a group, so that, the writing of check information is not concentrated on a single disk drive.
These RAID schemes show how data can be efficiently and reliably stored, but they do not consider the reliability of the channels by which the data are accessed. In the simplest interconnection scheme each disk drive is coupled to the host computer by its own channel. Besides requiring an impractically large number of channels for a large array, this scheme has the disadvantage that a failure in any channel disables access to one disk drive.
In another common scheme a sequence of disk drives is coupled to the host computer through a single channel. This scheme reduces the number of channels required, but has the disadvantage that the disks coupled to a given channel can be accessed only one at a time, and the further disadvantage that a channel fault disables access to some or all of the coupled disk drives.
In an elaboration of this scheme the channels are duplicated by providing two channels for access to each sequence of disk drives. This scheme greatly improves access reliability, but access is still limited to two disk drives per sequence at a time, and there is still the risk that faults in both channels may disable access to some or all of the disk drives in the sequence.